Beta molecular sieve is a molecular sieve having a high silica-to-alumina ratio is firstly synthesized in 1967 by Mobil Corporation, US using a classic hydrothermal crystallization method (U.S. Pat. No. 3,308,069). The unique structural characteristic of twelve-membered ring topology gives the Beta molecular sieve with unique catalytic performance and thus it is widely used in the petrochemical industry. It has similar pore size and adsorption capacity to the faujasite, but has a higher silica-to-alumina ratio, more excellent thermal and hydrothermal stability and stronger acidity compared with the faujasite. It can be used as a very effective active component for the preparation of catalysts in the processes of hydrocracking, isomerization, alkylation, catalytic cracking and the like. However, the high cost of synthesis has been one of the factors that limit the practical industrial application of Beta molecular sieve, so reducing the cost of synthesis has always been a key issue for the researchers in the field.
CN101439863A discloses a preparation method of Beta molecular sieve, in which a reaction mixture composed of solid alumino-silica gel, tetraethylammonium hydroxide, sodium hydroxide, a mother liquor of a synthesis product and a filtrate of the synthetic product after heat treatment with ammonia water is used as a raw material, wherein the ammonia water-treated filtrate of the synthetic product is obtained by filtering 2 to 5 wt. % of the ammonia water-treated product, Beta molecular sieve, at 70 to 100° C. for 0.5 to 4 h, and then the reaction mixture is hydrothermal crystallized at 120 to 170° C. for 3 to 7 days to produce the Beta molecular sieve. This preparation process greatly reduces the amount of tetraethylammonium hydroxide to reduce the production cost.
CN1116227C discloses the synthesis of a Beta molecular sieve by using an alkali metal ion-free system. Although this method omits the process of ammonium exchange, simplifies the process route to a certain extent and even improves the yield of the product, the amount of organic amine is increased significantly, which does not reduce the cost of production, but leads to a significant increase in the total cost. Also, exchange processes are often necessary to remove the residual metal ion impurities in the product molecular sieve, otherwise its catalytic performance would be seriously influenced, so the effect of reducing the cost cannot be realized in terms of process steps.
EP187522 discloses a method for the synthesis of Beta molecular sieve by uniformly mixing Na2O, SiO2, Al2O3 and a solid powder of Beta molecular sieve seed crystals into an aqueous solution of tetraethylammonium ion (TEA+), and producing the Beta molecular sieve via crystallization without external addition of water. The method reduces the production cost of the Beta molecular sieve and improves the yield to a certain extent, but the time for synthesis is still as long as up to 6 to 10 days, which is unacceptable for industrial production, and the relative crystallinity of the product is only 50 to 80%.
U.S. Pat. No. 5,164,169 discloses a synthesis technique of a large crystals of Beta molecular sieve in which the SiO2/Al2O3 molar ratio in the reaction system is generally greater than 70 and seed crystals of the large crystals of Beta molecular sieve are required to be externally added. Also, a large amount of organic nitrides is used with the use of a chelating agent, and thus the purpose of reducing the production cost is not achieved.
Landau (Chemistry of Materials, 1999, 11(8): 2030-2037) et al. synthesized Beta molecular sieve aggregates with particle size of 60 to 100 nm at low sodium ion content and a high concentration by using colloidal silica as a silica source and aluminum isopropoxide as an alumina source, and found that it was established by aggregation of smaller primary particles (10 to 20 nm).
All technical routes of Beta molecular sieve synthesis in prior art as described above uses inorganic chemical products as raw materials. These inorganic chemical products are mostly produced through complex reaction and separation processes from natural minerals, with long production process routes and high energy consumptions; and there is serious pollution emission in most of the processes, resulting in high production costs of these inorganic chemical products, and the production process are not environmentally friendly. The use of these products as raw materials for synthesizing the Beta molecular sieve would inevitably make the production cost at a high level. In order to solve the above problems, in recent years, the development of some technical routes in which natural clay minerals is used as the raw material to synthesize molecular sieves can reduce the cost for the molecular sieve synthesis to a certain extent.
CN101723398A discloses a method for synthesizing small crystals of Beta molecular sieve with montmorillonite, in which montmorillonite is subjected to acid treatment and high-temperature calcination. The acid-treated montmorillonite is heated and immersed in a template solution and serves as a raw material to provide all or part of the alumina source, and then is crystallized by adding a silica source and replenishing the alumina source, alkalis, water, an accelerator and a template agent to produce the small crystals of Beta molecular sieve under hydrothermal conditions.
CN101239321A discloses a montmorillonite/Beta molecular sieve composite and a method for producing the same. In this method, the montmorillonite/Beta molecular sieve composite was prepared by homogeneously mixing montmorillonite with Beta molecular sieve gel and then performing in situ crystallization and filtering, washing and drying. The process of this method is simple and the product is easy to be separated. The produced molecular sieve composite has good hydrothermal stability, but still needs to add silica and alumina sources in the form of inorganic chemical products.
Although the research of Beta molecular sieve synthesis by using natural clay minerals as raw materials has been paid attention, most of them still need additional silica and alumina sources in the form of inorganic chemical products. Only parts of silica and alumina in the minerals are used, and a certain amount of silica source or alumina source as inorganic chemical products are still required to be added to adjust the silica-to-alumina ratio, such that the silica and alumina sources as inorganic chemical products are not completely avoided. The technique of producing Beta molecular sieve having adjustable silica-to-alumina ratio from natural minerals as entire raw materials, without externally adding the silica and alumina sources as inorganic chemical products, has not been reported.
In recent years, with the development of green chemistry and chemical engineering, the use of non-toxic harmless raw materials, improvement of the utilization of raw materials, decrease of the energy consumption of the production process and reduction of pollution emissions have become the focus of new chemical process research and development. Beta molecular sieve is a widely used molecular sieve material in the field of petrochemical industry. If the silica and alumina sources in natural minerals can be utilized sufficiently and the silica-to-alumina ratio in the synthesis system can be adjusted without using inorganic chemical products as additional silica and alumina sources to produce the Beta molecular sieve, not only the source of raw materials is more extensive, but also can greatly shorten the synthesis route of raw materials to molecular sieves, significantly reduce energy consumption, material consumption and pollution emissions in the molecular sieve production process, and significantly reduce production cost, thereby providing a new green synthesis route for the synthesis of Beta molecular sieve, having broad prospects for development. However, the requirements for the synthesis condition of Beta molecular sieve is relatively high, and it is difficult to synthesize pure Beta molecular sieve by using chemical reagents in the prior art. By searching, no technical report about synthesis of the Beta molecular sieve having adjustable silica-to-alumina ratio by using natural minerals containing impurities as all of the silica and alumina sources was found.